Ingredients for the electronic nematic phase in FeSe revealed by its anisotropic optical response

The authors have found evidence from one single experiment of all relevant ingredients determining the anisotropy in the charge dynamics as well as in the dc transport properties of FeSe, a prominent iron-based superconductor. Scattering by spin fluctuations turns out to be the most decisive effect with respect to the anisotropic nematic properties. FeSe has lately acquired a central place among superconductors, since it harbors a key nematic phase below 90 K. This work describes novel results from reflectivity measurements, from the far-infrared up to the ultraviolet, that probe the optical response as a function of the tunable symmetry-breaking field (stress). The data suggest that the charge scattering rather than the Fermi surface parameters seems to be most influential with respect to nematicity. These findings may support theoretical proposals advocating a close connection between spin fluctuations, nematicity, and superconductivity.